The present description relates to a spark ignited internal combustion engine. It relates more specifically to a spark ignited internal combustion engine having relatively high compression ratio and to manufacturing such an engine.
It is known that a greater geometric compression ratio of an engine cylinder improves heat efficiency of an internal combustion engine. The geometric compression ratio is a ratio of a volume of a combustion chamber at bottom dead center during a cylinder cycle to a volume of the combustion chamber at top dead center during the cylinder cycle, and it can be said to be a geometric expansion ratio. Therefore, the greater geometric expansion ratio makes combusted gas in the combustion chamber expand more so that the heat energy can be converted more to mechanical energy output through the crankshaft of the engine.
When a spark ignited internal combustion engine operates, a spark plug ignites air-fuel mixture in a combustion chamber around top dead center of the compression stroke during a cylinder cycle, and then flame of the ignited air-fuel mixture propagates from a spark point of the spark plug through the un-combusted mixture toward a periphery of the combustion chamber. As the flame spreads and the combustion of the air-fuel mixture is completed more quickly, more heat is generated at an earlier stage of the expansion stroke. Then, the engine generates more torque resulting in a higher operating efficiency and lower fuel consumption of the engine.
Theoretically, the flame spreads in a radial direction from the point of the spark plug. The flame contacting a combustion chamber surface such as a piston head may deteriorate the flame propagation and delay the combustion completion or, in other words, a combustion period may be made longer. If the geometric compression ratio is greater, the combustion chamber volume at top dead center is smaller and the flame is more likely to contact to a combustion chamber volume.
One exemplary method to reduce the above described deterioration of the flame propagation is described in Japanese Patent Application Publication No. 2007-154827. A spherical cavity is formed on a piston head which defines a floor of the combustion chamber and is aligned with a spark plug gap when the piston is at top dead center during a cylinder cycle. The spherical cavity can delay a timing of the first contact between the flame and the piston head surface because the spherical cavity has its center coincide with the spark plug gap at the top dead center. Therefore, the combustion period can be shortened. Engine operating efficiency can be improved.
In accordance with the prior art method, a larger spherical cavity is more effective in terms of shortening the combustion period. But, the larger spherical cavity would result in a smaller geometric compression ratio if the cylinder volume were same. It would lead to lower operating efficiency of the engine.
Therefore, there is room to improve design of spark ignited combustion engines with higher compression ratios.